Method of making polishing material



mrnon or MAKING PoLisnrNo MATERIAL Cameron G. Harman, Cleveland Heights,Ghio, and

Ralph Rose, Ambler, Pa, assignors, by mesne assignments, to W. R. Grace& 60., New York, N.Y., a corporation of Connecticut No Drawing. Originalapplication Sept. 8, H59, Ser. No. 183,911, new Patent No. 2,744,601,dated May 1, 1956. Divided and this application Nov. 17, 1955, Ser. No.547,563

Claims. {CL 23-482) This invention relates to an improved polishingmaterial adapted for use in polishing glass or glass-like materials, andto a process of preparing such a polishing material.

Finely divided red iron oxide, known commercially as red rouge, has beenused in the polishing of glass and glass-like articles where a highlypolished glass surface is desired. The disadvantage of such a polishingmaterial, either in the natural form or when artificially prepared, isthat the polishing rate is rather slow. Moreover, red rouge is extremelydifficult material to use because it stains the person and clothing ofthe workers, and is generally obnoxious in the working area.

Certain siliceous materials, principally quartz, commercially availableas white rouge, have also been proposed as polishing materials for glasswhere a highly polished glass surface is desired. However, white rougehas a rather low polishing efiiciency, not equivalent to red rouge.

In addition, there are commercially available certain of the rare earthoxides, which are very good polishing materials, but which are quiteexpensive.

This invention is based upon the discovery that vitreous silica is anexcellent polishing material. Another aspect of this invention is thatmixtures of vitreous silica and rare earth oxides produce a polishingmaterial which is far superior to any polishing material now availableand which is unexpectedly superior to either of the two componentmaterials, when used alone. The abrasive power of a mixture of vitreoussilica and rare earth oxides is far superior to the abrasive power ofeither of these materials, singly, and vitreous silica alone has a muchbetter polishing efiiciency than white rouge or red rouge.

It is, therefore, one object of this invention to provide an improvedpolishing material.

It is another object of this invention to provide a method of makingsuch a polishing material.

It is still another object of this invention to provide a polishingmaterial which is economical to use, and which has a polishingefiiciency far superior to any commercially available polishingmaterial.

It is a further object of this invention to provide a method of makingsuch a polishing material.

Various additional objects and advantageous features of the presentinvention will become apparent upon reading the following description.

The chemical compound silica (SiO exists in four distinct forms. Theseforms are:

(1) Quartz.

(2) Tridymite.

(3) Cristobalite.

(4) Vitreous silica, which is an amorphous silica, and which is alsoknown as silica glass, fused silica, or fused quartz.

Silica, in the form of quartz, tridymite, or cristobalite, is acrystalline material, and though all three of these ?atented Feb. 25,196i forms of silica have the same chemical composition, each has a'dilferent crystal form, and each is distinguishable over the other bysuch properties as density, index of refraction, and crystal habits.

Quartz may exist as low-quartz or high-quartz. Tridymite may exist aslow-tridymite, high-tridymite, or upper high-tridymite. Cristobalite mayexist as lowcristobalite or high-cristobalite. Only low-quartz is stableat atmospheric temperature. Low-tridymite and lowcristobalite arecapable of existence at atmospheric temperatures, but are in ametastable state with respect to low-quartz. They tend to convert to thelow-quartz form of silica. This conversion to low-quartz is very slowand is not even measurable in terms of years. The lowquartz form is theform in which silica is usually found in nature.

Vitreous silica, which is an amorphous silica, is also known as silicaglass, fused silica, and fused quartz, Vitreous silica can exist atatmospheric temperatures, but is unstable at temperatures below about1710 C. Vitreous silica is characterized by isotropic (nondirectional)properties, low density, low thermal expansion, low index of refraction,and absence of crystallinity. Vitreous silica is characterized furtherby its lack of porosity or capil larity, as evidenced by nonpenetrationof dyes, and by the absence of water in its internal structure.

Unfortunately, he nomenclature of this art is somewhat indefinite. Manynaturally occurring deposits of silica, which on macro-inspection appearto be without regular crystalline form, are often referred to asamorphous silica, while in reality, such deposits of silica arecrypto-crystalline masses of low-quartz, thus, misleading the person notfamiliar with the materials, and/or the nomenclature.

While vitreous silica is an amorphous silica, the term vitreous silica,as used in this specification, does not include such crypto-crystallinemasses of low-quartz. Also, not to be included within the term vitreoussilica, as used herein, are other forms of silica sometimes referred toas amorphous silica, which come from both synthetic and natural sources,and which are similar to vitreous silica in some respects, but which arevery unlike it in other respects. For instance, as examples of materialsometimes referred to as amorphous silica, which are not to be includedwithin the term vitreous silica as used herein, are the finely dividedamorphous varieties, such as precipitated silica and naturally occurringhydrated amorphous silica.

The finely divided amorphous varieties, such as me cipitated silica, maybe prepared chemically from a basic silicate, such as sodium silicate,by the addition of hydrochloric acid. They may be deposited from avapor, for example, by passing silicon tetrafluoride vapors throughwater. Such amorphous varieties of precipitated silica usually containwater, and thus are different from vitreous silica as defined herein,which contains no water. Moreover, such amorphous materials asprecipitated silica, if heated high enough to remove their water, stilldiffer physically from vitreous silica by reason of their porosity, oneof their principal uses being that of a desiccant.

The naturally occurring hydrated amorphous silicas also difier fromvitreous silica. These hydrated silicas are usually classed as opal, oropaline varietiesof silica. This variety of silica, though by no meansrare in nature, constitutes a very small part of the naturally occurringsilicas with respect to the abundance of silica that occurs as thelow-quartz form. e

Naturally occurring hydrated amorphous silica differs from vitreoussilica in that it is usually much more porous than vitreous silica. Itis comparable to materials such as precipitated silica as to degree ofporosity.

The invention constituting a part of the subject matter of thisapplication is based upon the discovery that vitreous silica, as definedand limited herein, is an exceptionally fine polishing material.

As previously stated, vitreous silica has a high polishing efficiency ascompared with white rouge or red rouge. To illustrate the increasedpolishing efiiciency of vitreous silica as compared with red rouge andwhite rouge, the following example is given. It is to be understood thatthis example is illustrative in nature and should in no way be construedas a limitation on the scope of the disclosure herein.

Example I 50 grams of the polishing material in 100 cc. of water weregradually added to a 1% inch spectacle lens blank on a standardpolishing wheel. Every thirty seconds, the polishing was interrupted,the lens blank was removed, cleaned, and examined under a clear glassincandescent lamp. Starting with a spectacle lens in a fined condition,and using vitreous silica as the polishing agent, a highly polished,brilliant surface, containing no pits or lines, was obtained in sixminutes. Under the same test conditions, it took slightly more than tenminutes to produce a comparable polished surface with red rouge, andeven longer to produce a surface of the same quality, with white rouge.

Consequently, it should be apparent that vitreous silica is an effectivepolishing material which is far superior in polishing elficiency towhite rouge or red rouge. While vitreous silica is a superior polishingmaterial for polishing glass or glass-like surfaces, where a highlypolished surface is desired, and while this is one aspect of thisinvention, it has been discovered that mixtures of vitreous silica andrare earth oxides produce a polishing material that is superior toeither vitreous silica alone or the rare earth oxides alone.

To illustrate the unexpectedly superior polishing efiiciency of amixture of vitreous silica and rare earth oxides, the following exampleis given. It is to be understood that this example is merelyillustrative and should in no way be construed as a limitation on thescope of the disclosure herein.

The abrasive power is based on a wel known test for polishing materialsand represents the milligrams of glass removed from a 1% inch spectaclelens blank in minutes, using a standard M422A bowl feed polisher made bythe American Optical Company, with the gradual addition of 50 grams ofthe polishing material in 109 cc. of water.

Example II Polishing material: Abrasive power Vitreous silica 84 Rareearth oxides 96 64.5% vitreous silica-35 .5 rare earth oxides 165 Whilealmost any addition of the rare earth oxides to vitreous silica producessuch an unexpected increase in abrasive power and results consequentlyin an extremely fine polishing material, and while the combination ofthese two materials is considered to be the broad aspect of a part ofthis invention, it is preferred to incorporate rare earth oxides in thecombination of vitreous silica and cerium group rare earth oxides in anamount corresponding to, at least, by weight of the total mixture.Moreover, the preferred embodiment of a mixture of these two materialscomprises 35.5% rare earth oxides and the balance vitreous silica.

While any of the rare earth oxides or any combination of rare earthoxides, when added to vitreous silica, produce a polishing materialhaving such unexpected and improved polishing efiiciency, the rare earthoxides that are formed from the rare earths that naturally occur in Itis to be understood that any combination of vitreous silica and rareearth oxides as above defined is useful as a polishing material havingan unexpectedly improved polishing efiiciency. Moreover, while apreferred range has been set forth and a preferred exact composition hasalso been set forth, it is to be further understood that the mostpreferred embodiment of thecombination of vitreous silica and rare earthoxides includes limitations monazite sands are the preferred rare earthoxides. The

majority of such oxides is cerium oxide, while the balance comprisesvarious other rare earth oxides.

as to the particle size of these materials. Therefore, in all of theabove combinations of vitreous silica and rare earth oxides, or in thedisclosure relating to vitreous silica, per se, it is preferred that ofthe vitreous silica have a particle size of less than 5 microns, theremainder being between 5 and 20 microns in particle size.

it is also preferred that 60% of the rare earth oxides have a particlesize less than 5 microns, 15% of the rare earth oxides have a particlesize from 5 to 10 microns, and the remainder have a particle size from10 to 20 microns.

Vitreous silica, as defined herein, may be made in two ways. As a firstmethod, any of the various amorphous silicas may be heated in order toremove their water of hydration and eliminate their porosity so as toform vitreous silica. This is essentially a relatively low temperatureprocess. As a second method, vitreous silica, as defined and usedherein, may also be formed by heating any form of relatively pure silicaabove its melting point, and cooling the melted mass very rapidly toform vitreous silica, which is thereafter comminuted.

The first described method is the preferred method. The exacttemperature to which the amorphous silica in this method is heated andthe time that it is held at that temperature varies with the purity ofthe starting amorphous silica material. The temperature range, 'however,is between approximately 1860" F. and 2200 F. Depending upon the purityof the starting material, the exact time that the material is held at aparticular tem perature and the particular temperature employed in eachinstance are critical in producing an optimum quantity of vitreoussilica. For example, using a relatively pure amorphous silica, i.e.,silica gel, the optimum heat treatment was at 2000 F. for a period ofone hour. Above this temperature cristobalite tended to form. Below thistemperature, the silica gel did not entirely change to the desiredvitreous silica. When the starting material is relatively impure, thetime and temperature must be adjusted in order to produce an optimumquantity of vitreous silica. For example, in order to produce an optimumamount of vitreous silica, an impure silica gel which contained someflux was heated for a period of one hour at a temperature of 1900" F.This temperature and heat treatment produced the optimum amount ofvitreous silica from this type of material.

The polishing material comprising vitreous silica and rare earth oxidesmay be made in one of two ways. The vitreous silica and rare earthoxides may be mechanically mixed. However, a preferable method is to mixan amorphous silica and a salt of the rare earths and then calcine sucha mixture. This latter method is preferable because it produces apolishing material which, when used in polishing glass, results in theproduction of a slightly better finish on the glass article beingpolished.

In the performance of this second method, the raw materials arethoroughly mixed, wet or dry, to form a homogeneous batch. If thematerials are mixed wet, dewatering prior to calcination may be done byany of the methods commonly used in the chemical industry. The mixedmatch is then calcined at a temperature that will convert the amorphoussilica to vitreous silica and the salt of the rare earth to the rareearth oxide.

In the performance of this method, the preferred salt of the rare earthor rare earths employed is the basic sulfate. However, other salts ofthe rare earths such as the nitrates, oxalates, or others, which oncalcination will produce the rare earth oxide, are satisfactory. Aspreviously pointed out, the calcining temperature will vary, dependingupon the composition and purity of the material. However, it may begenerally stated that the calcining temperature should be high enough toconvert the salt of the rare earth to the rare earth oxide, but not sohigh as to form cristobalite from the amorphous silica startingmaterial.

From the foregoing, it should be apparent that there has been devised apolishing material which is extremely efiicacious for the polishing ofglass or glass-like objects where a highly polished surface is desired.This material, vitreous silica, moreover, when supplemented by theaddition of a rare earth oxide or a combination of rare earth oxidesproduces a polishing material having a polishing efiiciency, asevidenced by its abrasive power, far superior to the polishingefficiency of either vitreous silica alone or the rare earth oxidesalone.

Furthermore, novel methods of making such polishing materials have alsobeen disclosed.

While the above invention has been described in its preferredembodiment, it is to be understood that the words used are words ofdescription rather than of limitation, and that changes within thepurview of the appended claims may be made without departing from thetrue scope and spirit of the invention.

This application is a division of our co-pending application Serial No.183,911, filed September 8, 1950, now Patent No. 2,744,001.

What is claimed is:

1. The method of making vitreous silica suitable for use as a glasspolishing compound comprising heating amorphous silica to a temperatureof 18002200 F. for a time sufficient to drive off its water of hydrationand render the silica non-porous, rapidly cooling the heated silica andcomminuting the cooled silica to a particle size such that about 75% isless than 5 microns and the remainder is no greater than 20 microns.

2,. The method of making a glass polishing powder which comprisesheating amorphous silica to a temperature of 1800-2200 F. for about onehour, thereby rendering the silica vitreous, cooling said silica, andcomminuting said cooled material to a particle size below 20 microns.

3. The method of polishing glass which comprises rubbing said glass in aglass polisher with an aqueous slurry of vitreous silica having amaximum particle size of 20 microns and at least having a particle sizeof less than 5 microns, and continuing said rubbing until the desiredpolished surface is obtained.

4. The method of making of vitreous silica suitable for use as a glasspolishing agent comprising heating an amorphous silica to form a meltthereof, rapidly cooling said melt to form a water-free, non-porous massof silica and comminuting said cooled silica to a particle size lessthan 20 microns.

5. The method of making vitreous silica suitable for use as a glasspolishing agent comprising heating pure silica gel at a temperature ofabout 2000 F. for about one hour, whereby said silica gel is renderedwater-free and non-porous with a minimum of cristobalite formation,rapidly cooling said silica and comminuting the cooled mass.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Jacobs: The Abrasive Handbook, The Penton Pub. Co., 1928, p.430.

Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry,vol. 6, pages 288289.

1. THE METHOD OF MAKING VITREOUS SILICA SUITABLE FOR USE AS A GLASSPOLISHING COMPOUND COMPRISING HEATING AMORPHOUS SILICA TO A TEMPERATUREOF 1800-2200* F. FOR A TIME SUFFICIENT TO DRIVE OFF ITS WATER OFHYDRATION AND RENDER THE SILICA NON-POROUS, RAPIDLY COOLING THE HEATEDSILICA AND COMMINUTING THE COOLED SILICA TO A PARTICLE SIZE SUCH THATABOUT 75% IS LESS THAN 5 MICRONS AND THE REMAINDER IS NO GREATER THAN 20MICRONS.
 2. THE METHOD OF MAKING A GLASS POLISHING POWDER WHICHCOMPRISES HEATING AMORPHOUS SILICA TO A TEMPERATURE OF 1800-2200* F. FORABOUT ONE HOUR, THEREBY RENDERING THE SILICA VITREOUS, COOLING SAIDSILICA, AND COMMINUTING SAID COOLED MATERIAL TO A PARTICLE SIZE BELOW 20MICRONS.